This invention relates generally to an apparatus for polishing or planarizing a workpiece such as a semiconductor wafer, and more particularly, to a wear ring assembly for use on a polishing or planarizing apparatus that improves the uniformity of polishing and/or planarization.
The manufacture of many types of workpieces require the polishing and/or planarization of at least one surface of the workpiece. Examples of such workpieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. Without loss of generality, but for ease of description and understanding, the following description of the invention will focus on applications to only one specific type of workpiece, namely a semiconductor wafer. The invention, however, is not to be interpreted as being applicable only to semiconductor wafers. Those of skill in the art instead will recognize that the invention can be applied to any generally disk shaped workpieces.
Chemical mechanical polishing (CMP) is a technique which has been conventionally used for the planarization of semiconductor wafers. For example, see U.S. Pat. No. 5,099,614, issued in March in 1992 to Riarai et al; U.S. Pat. No 5,329,732 issued July 1994 to Karlsrud et al, and U.S. Pat. No. 5,498,199 issued March 1996 to Karlsrud et al. Furthermore, chemical mechanical polishing is often used in the formation of microelectriconic devices to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition. A typical chemical mechanical polishing apparatus suitable for planarizing a semiconductor surface generally includes a wafer carrier head configured to support, guide, and apply pressure to a wafer during the polishing process, a polishing compound such as a slurry to assist in the removal of material from the surface of the wafer, and a polishing surface such as a polishing pad.
A wafer surface is generally polished or planarized by moving the surface of the wafer to be polished relative to the polishing surface in the presence of a polishing compound. In particular, the wafer is placed in a carrier such that the surface to be polished is placed in contact with the polishing surface, and the polishing surface and the wafer are moved relative to each other while slurry is supplied to the polishing surface.
The construction of the carrier head and the relative motion between the polishing pad and the carrier head have been extensively engineered in an attempt to achieve a uniform rate of removal of material across the surface of the workpiece hence to achieve the desired planar surface. For example, the carrier head generally includes a flexible membrane that contacts the back or unpolished surface of the workpiece and accommodates variations in that surface. One or more pressure chambers may be provided behind the membrane so that different pressures can be applied to various locations on the back surface of the workpiece to cause uniform polishing across the front surface of the workpiece. The carrier head also generally includes a wear ring (sometimes referred to as a xe2x80x9cretaining ringxe2x80x9d or xe2x80x9cedge ringxe2x80x9d but hereinafter referred to without limitation as a xe2x80x9cwear ringxe2x80x9d) that surrounds the membrane and the workpiece and pre-stresses or pre-compresses the polishing pad to protect the edge of the workpiece. The wear ring, which has an internal diameter (ID) only slightly larger than the diameter of the workpiece, also serves to constrain the workpiece, maintaining the workpiece properly positioned under the carrier head. The polishing pad may move in a linear motion, a rotational motion, or an orbital motion, depending on the type of CMP apparatus being utilized. Additionally, the carrier head, and hence the workpiece, may also be in rotational motion. The relative motion between the carrier head and the polishing pad and the polishing pad itself are designed so as to hopefully provide the same degree of polishing to all areas of the polished side of the wafer. Unfortunately, uniform removal is not always obtained.
A CMP carrier head includes a wear or retaining ring around the periphery thereof forming a pocket or recess which receives and carries the wafer. The wafer-loaded carrier head is lowered and locked in a position such that the wafer is positioned and resides only a small distance above the polishing pad. Fluid pressure (e.g. air) is then applied to a diaphragm or bladder in the carrier head to push the wafer thru the remaining distance or gap between the wafer and the polishing pad and urge the wafer into the polishing pad. Without more, the edge of the wafer would dig into the polishing pad causing excessive polishing as the wafer""s edge, sometimes referred to as xe2x80x9cedge-bumxe2x80x9d. Thus, it is desirable that the polishing pad be as flat as possible in region of the wafer""s edge. To accomplish this, the wear ring, which positions and maintains the wafer within the carrier head, is also placed into contact with the polishing pad to compress (i.e. flatten) the polishing pad in the vicinity of the edge of the wafer. The wear ring is urged into contact with the polishing pad by means of a second diaphragm or bladder to which a fluid pressure (e.g. air) is applied.
The use of a wear ring for the purpose of flattening the polishing pad in the vicinity of the wafer""s edge presents certain challenges. First, if the depth to which the wear ring is pressed into the polishing pad is too low, edge-burn will still occur. If the depth is too high, the edge will be underpolished. In addition, if the material from which the wear ring is made is too soft (e.g. plastic), that portion of the wear ring adjacent the wafer""s edge may bend upward since the inner potion of the wear ring is thinner than the outer portion as will be more fully described hereinbelow.
The above described bending can be substantially avoided by using stiffer materials such as ceramic, steel, etc. This however presents a further challenge. Due to the geometry and configuration of existing CMP apparatus, the force applied to the wear ring assembly to urge it into the polishing pad is applied to an outer portion of the wear ring located at some distance from that portion of the wear ring under which the force must be realized (i.e. that region of the polishing pad substantially adjacent the edge of the wafer). Since the applied diaphragm pressure is not in line with that portion of the polishing pad at which the force must be realized, a twisting moment is induced into the wear ring assembly. This moment creates a toroidal deflection of the wear ring assembly, and the outer region of the wear ring exerts more pressure on the polishing pad than that region of the wear ring near its inner diameter (i.e. adjacent the wafer""s edge). The result is a loss of polishing control at the edge of the wafer. Furthermore, as the diaphragm pressure is varied, the induced moment is also varied resulting in a further loss of control.
In view of the foregoing, it should be appreciated it would be desirable to provide an improved wear ring assembly which substantially avoids the above referred to problems and improves polishing uniformity.
According to an aspect of the invention, there is provided a wear ring assembly for use in a workpiece polishing apparatus. The wear ring assembly comprises a wear element and a backing ring. The backing ring includes a fulcrum and is configured to transfer a pressure applied to the backing ring to the wear element via the fulcrum. The wear element is retained by means of a clamp ring coupled to the carrier head of the polishing apparatus.